The storage and transmission of full-color, full-motion images is increasingly in demand. These images are used, not only for entertainment, as in motion pictures or television shows, but also for complicated analytical and diagnostic tasks such as engineering analysis and medical imaging.
There are several advantages to providing these images in digital form. The images are more susceptible to enhancement and manipulation and they are more readily indexed for storage and retrieval. As with all digital representations, digital video images can be regenerated exactly. Additionally, computer and television technologies are on the threshold of merging. Both fields will benefit from the resulting synergies, but in order to take full advantage of this merger, television will shift to the digital technology which dominates computer technology. Many of the advantages of digital television and commonly used data compression techniques are discussed in chapters 18 and 19 of a reference entitled "Television Engineering Handbook", K. Blair Benson, Editor in Chief, McGraw-Hill Book Company, 1986 which is hereby incorporated by reference.
There are two major problems associated with the use of digital images; they require an immense amount of memory for storage and they consume tremendous channel bandwidth when they are transmitted. For example, a single 512 by 512 pixel gray-scale image with 256 gray levels requires greater than 256,000 bytes of storage. A full color image requires nearly 800,000 bytes. Natural-looking motion requires that images be updated at least 30 times per second. The transmission channel for natural-looking full color moving images must therefore accommodate approximately 190 million bits per second. One minute of full color video requires almost 2 Gigabytes of storage.
As a result of the increasing demand for color digital video images and because of the huge potential cost savings, many image compression techniques have been employed to reduce both the transmission bandwidth and storage area required by digital video signals. These techniques generally take advantage of the fact that there is a great deal of redundancy in any natural image and, fortunately, the human psycho-visual system does not respond to abrupt time-based or spatial transitions. This permits the use of both time-domain and spatial-domain techniques to reduce the amount of data used to transmit, record, and reproduce color digital video images.
For example, differential pulse-code modulation (DPCM) is a commonly used compression technique which relies upon the facts that video images, generally, are quite redundant and that any transitions in the images are, for the most part, gradual. A DPCM encoder will therefore predict upcoming pixel values based upon previous pixel values then compare the actual value with the predicted value to obtain an error signal. The error signal is the encoded value. If the predictions are relatively accurate, the error signal will occupy a great deal less memory and/or bandwidth than the original video signal. The signal can be decoded by using the prediction algorithm in conjunction with the error signal.
Other compression techniques further rely upon the fact that the human psycho-visual system is less sensitive to changes in chrominance than to changes in luminance. In one exemplary embodiment these systems undersample the chrominance information in a video signal by dividing the image into an array of n-pixel by n-pixel squares and assigning a single chrominance value to each of the squares. This reduces the chrominance information that must be stored or transmitted by a factor of n-squared.
Unfortunately, few natural images are composed of square chrominance regions and the size of the squares used in undersampling the chrominance information must be kept small in order to prevent "bleeding" of color from region to region. Bleeding is especially common in those areas of an image which exhibit the greatest spatial color variation. As a result, those regions tend to set the lower bound on the chrominance sampling frequency which may be employed for the image. But setting the sampling frequency high enough to preclude bleeding even in the areas of greatest spatial color variation reduces the compression ratio that may be obtained.
It is therefore an object of the invention to improve the image quality of compressed color video images, specifically, to reduce the amount of "bleeding" attributable to a chrominance-compression system, and to increase the compression of the images.